With natural gas reserves continuing to increase, U.S. engineers are testing ways to convert methane gas economically into building blocks to produce chemicals, a job traditionally handled by petroleum.
“With petroleum reserves in decline, natural gas production is destined to increase to help meet worldwide energy demands,” said University of Virginia chemical engineering professor Matthew Neurock. “But petroleum — in addition to being used to make fuels — is also used to make ethylene, propylene and other building blocks used in the production of a wide range of other chemicals. We need to develop innovative processes that can readily make these chemical intermediates from natural gas.”
However, there yet are no cost-effective ways to do this, he said. Because methane “is rather inert,” it requires high temperatures to activate chemical bonds and its conversion to “useful chemical intermediates” so far has eluded discovery.
Dow Chemical in 2007 issued a “methane challenge” to seek new processes to convert methane to ethylene and other useful chemicals. The chemical company received “about 100 proposals from universities, institutes and companies around the world,” and research grants were given to Northwestern University and Cardiff University.
The Northwestern team, which Neurock is working with, is using theoretical methods and high-performance computing to understand the processes that control catalysis and to guide the experimental research, which recently was detailed in the journal Nature Chemistry.
Using sulfur as a “soft” oxidant, the chemists and engineers are testing ways to convert methane into ethylene, a key chemical intermediate used to manufacture chemicals, polymers and fuels and possibly films, surfactants, detergents, antifreeze and textiles.
“We show, through both theory — using quantum mechanical calculations — and laboratory experiments, that sulfur can be used together with novel sulfide catalysts to convert methane to ethylene, an important intermediate in the production of a wide range of materials,” Neurock said.
The chemists and engineers had attempted to develop catalysts and catalytic processes that use oxygen to make ethylene, methanol and other intermediates, but they have had little success because oxygen “is too reactive and tends to over-oxidize methane to common carbon dioxide.”
Sulfur and other “softer” oxidants with weaker affinities for hydrogen could be the answer because they may help to limit the overreaction of methane to carbon disulfide, Neurock said.
“In the team’s process, methane is reacted with sulfur over sulfide catalysts used in petroleum processes. Sulfur is used to remove hydrogen from the methane to form hydrocarbon fragments, which subsequently react together on the catalyst to form ethylene.”
The collaborators have found that theoretical and experimental results “indicate that the conversion of methane and the selectivity to produce ethylene are controlled by how strong the sulfur bonds to the catalyst.” With these concepts in place, they have begun exploring how different metal sulfide catalysts “ultimately tune the metal-sulfur bond strength” to control methane’s conversion to ethylene.
“The abundance of natural gas, along with the development of new methods to extract it from hidden reserves, offers unique opportunities for the development of catalytic processes that can convert methane to chemicals,” Neurock said.
“Our finding — of using sulfur to catalyze the conversion of methane to ethylene — shows initial promise for the development of new catalytic processes that can potentially take full advantage of these reserves. The research, however, is really just in its infancy.”
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